1. Field of the Invention
The present invention is directed to a control circuit device, particularly for a gradient system of a magnetic resonance apparatus.
2. Description of the Prior Art
Control circuit devices are widespread in nearly all areas of technology, including magnetic resonance technology. For realizing controllers with digital means, for example, FIG. 3.12 in the book by G. Schmidt, xe2x80x9cGrundlagen der Regelungstechnikxe2x80x9d, Berlin, Springer-Verlag, 1987, pp. 173-180, shows a schematic signal flow diagram of a digital controller. Further, a shift of an addition place within a signal flow diagram relating to the signal flow image algebra is described in the same book on pages 92-96. In base algorithms for digital controls, further, FIG. 11.2-10 in the book by H. Lutz and W. Wendt, xe2x80x9cTaschenbuch der Regelungstechnikxe2x80x9d, Frankfurt am Main, Harri-Deutsch-Verlag, 1998, pages 417-427, shows a control circuit wherein an analog controlled quantity is fedback digitally converted onto an input of the control circuit.
In a magnetic resonance apparatus, a gradient field generated by a gradient system, which contains at least one gradient coil and a gradient amplifier, is rapidly switched. To that end, a gradient current generated by the gradient amplifier for the gradient coil is correspondingly regulated with a control circuit device. German OS 198 37 440 discloses a control circuit device for this purpose. In the control circuit device of this published application, a command variable generator generates a digital command variable that is supplied to a high-precision digital-to-analog converter for generating an analog command variable. Together with an analog measured quantity of the gradient current as a regulating variable, the analog command variable is supplied to an analog differential amplifier for forming an analog actuating variable quantity. The analog actuating variable quantity is supplied to an analog integrator for forming an analog, integrated actuating variable quantity. The analog, integrated actuating variable quantity is supplied to an analog-to-digital converter for forming a digital, integrated actuating variable quantity. The digital, integrated actuating variable quantity, finally, is supplied to a controller of a gradient amplifier, which emits the gradient current as a controlled output quantity.
Especially problematical in the above-described control circuit device is that the analog integrator tends to drift as a consequence of offset voltages and the high-precision digital-to-analog converter is not linked into the actual control loop, so that its imprecisions are not equalized.
An object of the present invention is to provide an improved control circuit device, particularly for a gradient system of a magnetic resonance apparatus, that, among other things, avoids the aforementioned disadvantages of known devices.
This object is inventively achieved in an inventive control circuit device, particularly for a gradient system of a magnetic resonance apparatus, having a command variable transformer to which a command variable is supplied and that is configured for generating a first output quantity that corresponds to a time integration of the command variable, a regulating variable transformer to which a regulating variable is supplied and that is configured for generating a second output quantity that corresponds to a time integration of the regulating variable, a comparator to which the two output quantities are supplied and that is configured for generating an actuating variable quantity from the output quantities, and a controller or regulator to which the actuating variable quantity is supplied and that is configured for generating a manipulated variable with which the regulating variable can be controlled or regulated via a controlled system.
In, for example, a magnetic resonance apparatus, a gradient current/time integral is employed as a regulating variable as a result so that the gradient current/time integral can be directly controlled as the quantity to which a spin ensemble to be investigated primarily reacts.
In an embodiment, the regulating variable transformer is a nuclear magnetic spin or electron spin magnetometer. To that end, at least parts of the nuclear magnetic spin or electron spin magnetometer are arranged within an electrical coil to which the regulated variable is supplied for generating a magnetic field.
The nuclear magnetic spin or electron spin magnetometer for, among other things, measuring a magnetic field which is adapted to receive a specimen of a material that produces nuclear magnetic resonance or electron spin resonance having a resonant frequency that is dependent on a magnetic flux density of the magnetic field to be measured, and has a transmission device for emitting a transmission signal into the specimen with at least one prescribable transmission frequency that has a frequency spacing from the resonant frequency, and a reception device for receiving a mixed signal with mixed frequencies containing the resonant frequency and the transmission frequency and for filtering out the resonant frequency from at least one of the mixed frequencies as a criterion (indicator) for the magnetic flux density.
A spin resonance of the specimen is thereby used as a non-linear component. The essentially fixed transmission frequency thus can be prescribed such tat the utilized mixed frequency of the mixed signal can be filtered out by a broadband filter having a short transmit time. A signal oscillating at the resonant frequency that represents an indicator or identifier for the magnetic flux density to be measured can be ultimately acquired by a following mixing of the filtered mixed signal with a signal oscillating with the transmission frequency. Among other things, magnetic fields that change arbitrarily fast in time thus can also be measured. A re-adjustment of the transmission frequency is not necessary, and therefore a control device for such readjustment is not necessary.
In an embodiment, the reception device of the magnetometer has a counter with which cycles of a signal that oscillates at the resonant frequency can be counted, and the counter is fashioned to emit a counter reading that is one indicator for the electrical charge of a current that corresponds to the magnetic field to be measured. When the specimen of the magnetometer is arranged within an electrical coil in which this current flows, a current-time integral of the current can be directly measured and is available as a digital quantity as the counter reading emitted by the counter.
In another embodiment, the transmission device of the magnetometer has a phase shifter for generating at least a 180xc2x0 phase shift of the transmission signal. This 180xc2x0 phase shift can thereby be generated either following a prescribable time duration or dependent on the amplitude of the mixed signal. As a result, the signal amplitude of the mixed signal, that decreases over time, is maintained at a relatively high level by re-exciting the spins in the specimen, so that a consistently high signal-to-noise ration can be achieved. In particular, the generation of 180xc2x0 phase shifts dependent on the amplitude of the mixed signal has the advantage that changes in the T2 decay time of the specimenxe2x80x94due, for example, to field inhomogeneities of the magnetic field to be measuredxe2x80x94can be dynamically adapted.
In a further embodiment, the transmission device of the magnetometer is fashioned such that a magnetization amplitude of the transmission signal is smaller by factor of approximately 10xe2x88x923 than the magnetic flux density to be measured. As a result, influence of the magnetization amplitude on the resonant frequency is negligible, so that it is not necessary to make a correction by a frequency component corresponding to the magnetization amplitude to be subsequently implemented for the resonant frequency that has been filtered out.
In another embodiment, the nuclear magnetic spin or electron spin magnetometer has a magnetic field generator for generating a static magnetic field the pre-polarizes the specimen. As a result, a magnetic flux density with a value of zero can be unambiguously identified and detected with the magnetometer.